Detection of particles and colloids suspended in a fluid medium for measurement of concentration or other properties is useful in a variety of applications such as medical diagnostics, scientific research, air quality measurements, and threat detection. An example apparatus for determining carbon particle concentration in combustion exhaust is described in U.S. Pat. No. 8,531,671. Another example system for material analysis is described in U.S. Pat. No. 8,411,272. Examples include measurement of the concentration of airborne particles in inside environments such as buildings as well as outside environments.
One application of note is the measurement of the concentration and other properties of airborne particles (or particulate matter, PM) in aerosols. The United States Environmental Protection Agency (US EPA) has set exposure standards for coarse PM (between 10 μm and 2.5 μm, PM10) and fine PM (less than 2.5 μm, PM2.5) due to the importance of aerosol concentration in the air and its health effects. Aerosol concentrations are also important in the manufacturing industry for both protection of the health of workers and preventing contamination in the manufacturing process.
A class of aerosols of special interest is bioaerosols. Bioaerosols include bio-particles such as fungus spores, bacteria spores, bacteria, viruses, and biologically derived particles (skin cells, detritus, etc.). Some bioaerosols cause chronic and/or acute health effects, for example certain strains of black mold or Bacillus anthraces (causative bacteria of anthrax). Bioaerosol concentrations are important in maintaining safe hospitals, clean food processing, pharmaceutical and medical device manufacturing, and air quality. Airborne spread of diseases is of particularly concern from a public health perspective. Aerosolized bioagents can also be used by terrorists to harm civilian or military populations.
Measurement (sensing) of aerosol and bioaerosol concentration is typically accomplished with optical techniques. Aerosol (e.g., solid and liquid particles ≤10 μm dispersed in air) concentration measurement is readily achieved by various light scattering measurements. The most accurate method entails the use of a single particle counter that focuses a stream of aerosol into a detection cavity where light scattering from a long wavelength (>650 nm) laser is measured. Precision optics are required to collect and focus the scattered light (while excluding the source light) onto a photon detector. The photon detectors are made from silicon or photocathode materials (e.g., indium gallium arsenide) that undergo the photoelectric effect (convert photons to electrons). These materials are packaged into detectors that offer high amplification of the signal from the photons, such as photomultiplier tubes (PMTs) and avalanche photodiodes (APDs). These detectors have active detection areas that are small (less than 25 mm2) and limited to planar geometries. Moreover, these detectors cost $100 or more, often exceeding $1,000 in the case of a high sensitivity PMT.
Autofluorescence (or intrinsic fluorescence) excited by ultraviolet (UV) and blue light is well-developed for detection of bioaerosols. See Hairston et al., “Design of an instrument for real-time detection of bioaerosols using simultaneous measurement of particle aerodynamic size and intrinsic fluorescence,” Journal of Aerosol Science 28(3): 471-482 (1997); Ho, “Future of biological aerosol detection,” Analytical Chimica Acta 457(1): 125-148 (2002); Agranovski et al., “Real-time measurement of bacterial aerosols with the UVAPS: Performance evaluation,” Journal of Aerosol Science 34(3): 301-317 (2003); Ammor, “Recent advances in the use of intrinsic fluorescence for bacterial identification and characterization,” Journal of Fluorescence 17(5): 455-459 (2007); Ho et al., “Feasability of using real-time optical methods for detecting the presence of viable bacteria aerosols at low concentrations in clean room environments,” Aerobiologia 27(2): 163-172 (2011). Exploiting autofluorescence of microbes is widely viewed as one of the most cost-effective means to detect a potential biological threat. Bioaerosol detectors typically use a combination of light scattering (measurement of general aerosol concentration and properties) and autofluorescence (detection of emitted photons). Bioaerosol detectors based on autofluorescence rely on fluorescence from molecular fluorophores that reside within the bio-particle. For clean bio-particles, this fluorescence can be primarily attributed to biochemicals such as tryptophan and tyrosine (amino acids), nicotinamide adenine dinucleotide (NADH), and riboflavin. NADH and riboflavin absorb and emit longer wavelengths than the amino acids. See Jeys et al., “Advanced trigger development,” Lincon Laboratory Journal 17(1): 29-62 (2007); Hill et al., “Fluorescence of bioaerosols: mathematical model including primary fluorescing and absorbing molecules in bacteria,” Optics Express 21(19): 22285-22313 (2013). The ability to use longer wavelength excitation sources such as light-emitting diodes (LEDs, excitation wavelength λexc>360 nm) or lasers (λexc>400 nm) may reduce the cost of such instruments.
Traditional bioaerosol particle detectors rely on three main components: (1) an excitation source of appropriate wavelength to excite a targeted fluorophore or collection of fluorophores; (2) precision optics (lenses and mirrors) on both the excitation and emission side to focus the source onto the narrow air stream and to enhance the collection of emitted photons from biological particles; and (3) a high gain detector such as a PMT or APD. Elastic light scattering from visible or long wavelengths is utilized to count and sometimes size the particles. Autofluorescence of biomolecules is utilized to detect microorganisms. The typical bioaerosol detector utilizes a small detection cavity, with fluorescence active volumes on the order of 1×10−4 cm3, making the window for detection of each bioaerosol particle exceedingly small. At typical flow rates, a bioaerosol particle resides within the excitation volume for 1-10 μs on average. See Hairston et al. (1997). As a result, emitted and scattered light from each bioaerosol particle is collected virtually on an individual basis, and the signal is weak. See Greenwood et al., “Optical Techniques for Detecting and Identifying Biological Warfare Agents,” Proceedings of the IEEE 97(6): 971-989 (2009). This weak signal thus requires the use of precision lenses and mirrors to collect the weak signal and focus it onto the high gain detector (e.g., PMT or APD).
Measurement of aerosol and bioaerosol concentration and changes in concentration is possible via a variety of commercially available instruments such as the Laser Aerosol Spectrometer for aerosols (TSI Incorporated, Shoreview, Minn., USA), the Ultraviolet Aerodynamic Particle Sizer for bioaerosols (TSI Incorporated), the Wideband Integrated Bioaerosol Sensor (WIBS-4) for bioaerosols (Droplet Measurement Technologies, Boulder, Colo., USA), and the instantaneous biological analyzer and collector (FLIR Systems, Inc., Wilsonville, Oreg., USA). However, such instruments can exceed $10,000 in cost making wide spread use cost prohibitive. Furthermore, having a sufficiently dense sensor network of aerosol/bioaerosol sensors (i.e., multiples of these instruments in communication with a central network) is cost prohibitive. The high cost of a sensor network also means that capitalizing on responsive systems is challenging. For example, it would be desirable to provide several bioaerosol sensors positioned throughout a hospital or other building and networked with the building's control systems to maintain a safe environment and respond to a change in bioaerosol concentration, such as by diverting airflow or indicating the need for maintenance of filters and air handlers.
Aerosol exposure monitors have been developed that acquire data from aerosol while the aerosol is sampled in real time during a prescribed sampling period (integration period). Such devices may employ inertial impactors for aerodynamic sizing, particle collection filters for collection and subsequent analysis, and nephelometers for measuring particle concentration by acquiring light scattering data in real time. Examples of such devices are described in International Publication No. WO 2013/063426, filed Oct. 26, 2012, titled “AEROSOL EXPOSURE MONITORING,” the content of which is incorporated by reference herein in its entirety. Also known are turbidometers, which measure the concentrations of particles such as cells in solution.
In collecting air samples for the purpose of air quality monitoring, maintaining a constant flow rate is important. However, as a filter loads with particles, the pressure drop increases and results in reduced flow. This changing of the flow rate with time can lead to inaccurate quantification of the amount of aerosol or other sample present. Accordingly, there is a need for systems and techniques for managing flow rate.